Nutrient factors and dietary needs are now recognized as specific modifiers of mammalian gene expression. One of the major factors associated with nutrient regulation is the availability and transport of amino acids. The liver represents a major organ for amino acid metabolism, and also is the site for synthesis of important plasma proteins such as albumin. As a result, control of amino acid metabolism and protein synthesis in response to amino acid availability is critical for hepatic function. Interestingly, given the portal circulation from the intestine, the liver is also subjected to wider fluctuations in circulating amino acids than any other post-absorptive organ. My hypothesis is that hepatocytes respond to fluctuations in amino acid availability through a signalling pathway to regulate metabolism. The immediate goal is to identify the steps involved in amino acid-dependent control of transcriptional and posttranscriptional mechanisms used to regulate mRNA content in liver cells. My long-term goal is to characterize the entire amino acid signalling pathway, to better understand how mammalian cells respond to their environment through metabolite control of gene expression. The proposed studies will investigate the transcriptional control of the ribosomal protein genes for L17 and S25, both of which exhibit increased mRNA levels after amino acid deprivation. Interestingly, the elevated levels of mRNA are restricted to the nucleus and not exported for storage and/or translation. We will investigate the mechanisms responsible for this nuclear retention. In addition, a genomic clone for either L17 or S25 will be isolated and amino acid-response elements within the gene identified. Genomic characterization will include nuclear run-off assays, deletion analysis, in vivo footprinting, and in vitro band shifts assays. Defining the molecular steps by which amino acids can regulate gene expression may provide a general model for the molecular description of metabolite control in all mammalian cells.